This project will study in detail the growth and development of postnatal human skeletal muscle in vitro. Tissue culture techniques will be combined with light microscopy of growing culture, histochemical studies of different stages of muscle growth, and ultrastructural studies of newly grown muscle, to provide basic information about growth characteristics of human skeletal muscle. Initially, histologically normal skeletal muscle will be studied, as a necessary prerequisite to subsequent study of histologically abnormal material. Attempts to identify differences in growth characteristics, histochemistry, and ultrastructure will be made comparing newly grown muscle from normal and abnormal material.